Medical electrical leads are used in conjunction with numerous types of medical devices for monitoring the electrical activity of and/or stimulating excitable body tissue. Such devices include cardiac pacemakers, cardiac defibrillators, cardioverters, myostimulators, neurostimulators, and other devices for delivering electrical signals to excitable body tissue and/or receiving electrical signals from the tissue. Cardiac rhythm management devices, for example, including pacemakers, cardioverters and defibrillators, are designed to operate so as to sense the intrinsic cardiac electrical activity and deliver appropriately timed electrical stimulation signals when needed, in order to maintain a normal sinus rhythm at a physiological heart rate. The overall performance of these devices depends largely on the performance of the associated lead system.
Medical electrical leads typically bear one or more electrodes located near a distal lead end that is positioned in the vicinity of body tissue targeted for sensing and/or stimulating. An electrical conductor extends between each electrode and a connector provided at the proximal lead end for electrically coupling the lead and its electrode(s) to a medical device. Medical lead conductors are typically insulated, metallic wire based conductors. A number of limitations exist, however, in using medical electrical leads that rely on metallic wire conductors for carrying sensed electrical signals from excitable body tissue to an implanted or external device.
One limitation is the presence of electrical noise, which may be in the form of electromagnetic interference or electrical potential signals arising from other nearby excitable tissue, sometimes referred to as “far-field signals”. Such noise or far-field signals contaminate the sensed signal, interfering with the detection of electrical signals of interest. In regard to cardiac rhythm management devices, accurate sensing of intrinsic cardiac events is crucial to device performance. Intrinsic cardiac events of interest can include atrial depolarizations, observed as P-waves on an internal cardiac electrogram (EGM) signal, and ventricular depolarizations, observed as R-waves on an EGM signal. Oversensing or undersensing of these intrinsic events by a cardiac rhythm management device can result in incorrect detection and classification of a rhythm (normal versus pathological), potentially triggering the delivery of unnecessary cardiac stimulation therapy or inappropriately withholding stimulation when it is actually needed.
Inappropriate withholding of cardiac stimulation is undesirable when the patient is pacemaker dependent or the stimulation therapy is life saving. Inappropriate delivery of cardiac stimulation is undesirable because it may cause unnecessary pain to the patient if the therapy delivered is a shock, and can also lead to premature device battery depletion. Moreover, delivery of stimulation therapies in the presence of normal intrinsic cardiac activity can result in stimulation pulses being delivered during the so-called “vulnerable period” of the cardiac cycle, during which cardiac arrhythmias are easily induced in some patients, creating a potentially life-threatening situation.
The vulnerable period immediately follows the repolarization of cardiac cells after a depolarization. The repolarization time of cells located at the stimulation site is difficult to ascertain from EGM signals sensed using metallic wire based leads because EGM signals reflect the summation of many cellular action potential signals as a depolarization wavefront moves through the myocardium. The EGM signal does not resemble an action potential signal and therefore the recovery time of local cells cannot be accurately estimated from an EGM. Moreover, the T-wave, which contains the repolarization information in a far-field EGM, can have complex morphology making it difficult to ascertain exact repolarization time and characteristics using signal processing techniques. To avoid delivering electrical stimulation during the vulnerable period, some stimulation therapies, such as anti-tachycardia pacing therapies, are synchronized with ventricular depolarization and therefore rely on accurate R-wave detection. A more reliable approach to avoiding the vulnerable period, however, would be to sense the local repolarization of cardiac cells at the stimulation site. Therefore it is desirable to provide a medical lead for sensing the entire action potential morphology from which local activation and recovery times can be easily and accurately measured.
To minimize the likelihood of oversensing or undersensing intrinsic cardiac events, special sensing circuitry, such as sense amplifiers having automatically adjustable sensitivity and gain levels and various sense amplifier blanking schemes have been developed. However, despite these improvements, noise and far-field signals remain an infrequent but serious problem that undermines the accuracy of cardiac event sensing using metallic wire based medical electrical leads.
Another limitation encountered with metallic wire based lead systems relates to sensing of an evoked response following a cardiac pacing pulse. During cardiac pacing, evoked response sensing is performed in order to verify that a delivered pacing pulse has depolarized, or “captured,” the heart. A pacing threshold search can be performed to determine the minimum pulse energy needed to capture the heart, referred to as the “pacing threshold.” During a pacing threshold search, the evoked response is detected following pacing pulses of varying pulse energies in order to determine the pacing threshold. Pacing at a pulse energy just above the pacing threshold (i.e., threshold+a fixed safety margin) is desirable in order to ensure capture while preserving device battery longevity.
During normal pacing operations, capture management schemes typically employ evoked response sensing to verify that capture is not lost due to a change in pacing threshold. False capture detections due to oversensing of noise or far-field signals may result in prolonged episodes of subthreshold cardiac pacing that is ineffective in maintaining a base heart rate. False loss of capture detections can result from undersensing of the evoked response and can trigger the delivery of unnecessary backup pacing pulses and pacing threshold searches. Increases in pacing pulse energy due to false loss of capture detections can lead to premature pacemaker battery depletion. Accurate capture verification and maintenance of effective cardiac pacing therefore depends on reliable evoked response sensing.
Evoked response sensing using metallic wire based leads is difficult, however, for a number of reasons. A major challenge in evoked response sensing arises due to the post-pace polarization artifact at the electrode-tissue interface. This polarization artifact, also referred to as “afterpotential,” can saturate sense amplifiers included in the cardiac pacing device and mask an evoked response signal. Typically, a blanking interval is applied to sense amplifiers during and immediately following a pacing pulse to prevent saturation of the amplifiers. The polarization artifact may diminish during the blanking interval, however, it may still interfere with evoked response sensing. Low-polarization electrodes have been proposed for reducing the polarization artifact. See for example U.S. Pat. No. 4,502,492, issued to Bornzin, or U.S. Pat. No. 6,430,448, issued to Chitre, et al.
Improved methods for performing capture verification based on evoked response sensing using conventional leads have been proposed. Such methods may include special hardware circuitry or special software signal processing methods that reduce or eliminate the problem of polarization artifact. Reference is made to commonly assigned U.S. Pat. No. 6,134,473, issued to Hemming et al. and U.S. Pat. Application No. 20020116031 issued to Vonk.
Selection of separate sensing electrodes for sensing the evoked response, different than the electrode pair used for delivering the pacing pulse, can reduce polarization artifact problems. Other methods proposed for overcoming post-pace polarization artifact during capture verification include sensing a far-field signal related to an evoked response, as opposed to the near-field evoked response signal, or sensing a conducted polarization away from the pacing site. See for example, U.S. Pat. No. 5,324,310 issued to Greeninger, U.S. Pat. No. 5,222,493 issued to Sholder, U.S. Pat. No. 5,331,966 issued to Bennett et al., U.S. Pat. No. 6,434,428 issued to Sloman, et al., and U.S. Pat. App. No. 20010049543, issued to Kroll.
For accurate evoked response detection, however, it is desirable to sense the evoked response in the vicinity of the stimulated cardiac tissue site. Sensing in other areas of the heart could lead to erroneous evoked response detection due to noise or other myopotentials being sensed as an evoked response. Furthermore, sensing for an evoked response conducted to another area of the heart may not be possible in patients having conduction disorders. A medical sensing lead that is not subject to electrical noise, far-field signals or post-pace polarization artifact is therefore desirable for use with cardiac rhythm management devices in order to achieve reliable, accurate sensing of intrinsic and evoked electrical activity.
Accurate sensing of intrinsic electrical activity is important in other diagnostic and therapy delivery applications. Electrophysiological studies are performed on the heart to identify patients that are prone to arrhythmias, and identify and treat arrhythmogenic substrate. Accurate detection of electrical activation and recovery is valuable in understanding the potential for arrhythmias and the pathways by which an arrhythmia originates and is sustained. Therefore, detection of a local action potential signal, rather than a relatively more global EGM signal, would provide more detailed information regarding activation and recovery times of myocardial cells. Detection of local action potential signals would also be valuable in monitoring the effect of pharmaceutical agents on cellular activation and recovery.
In certain cardiac pacing therapies, it is desirable to time the delivery of the pacing pulse relative to myocardial repolarization at the stimulation site. Such therapies include anti-tachycardia pacing, cardiac potentiation therapy based on post-extrasystolic potentiation, and non-excitatory stimulation. During anti-tachycardia pacing, avoiding delivery of a pacing pulse during the vulnerable period is critical in preventing a worsening of the arrhythmia. Detection of approximate local repolarization time based on T-wave sensing is generally disclosed in U.S. Pat. No. 4,593,695 issued to Wittkampf for use in timing the delivery of anti-tachycardia pacing relative to the sensed T-wave.
Post-extrasystolic potentiation (PESP) refers to the enhanced mechanical function of the heart following an early extrasystole. The magnitude of the enhanced mechanical function is strongly dependent on the timing of the extrasystole. Because the extrasystole is most effective just after repolarization, a perceived risk in delivering PESP stimuli is that the extrasystole may fall in the vulnerable period. A post-extra systolic potentiation cardiac pacing stimulator for applying paired or coupled pulses is generally disclosed in U.S. Pat. No. 5,213,098, issued to Bennett et al.
Non-excitatory stimulation (NES) is delivered to cardiac tissue while it is undergoing active depolarization and repolarization to influence electrochemical and electromechanical dynamics in order to modulate cardiac contractility. A method for automatically controlling the delivery of excitable tissue control signals that includes the determination of an estimated action potential duration is generally disclosed in U.S. Pat. No. 6,360,126 issued to Mika et al. The action potential duration is estimated from an action potential related signal which, in a preferred embodiment of the invention, is a close bipolar electrogram signal.
For each of these types of cardiac pacing therapies, therefore, accurate detection of myocardial repolarization would be advantageous in properly timing the delivery of the stimuli. An experimental system for recording high-fidelity transmembrane action potentials using an optical mapping system and voltage-sensitive dye is described by Laurita, et al., Circ. Res. 1996. Methods and apparatus for measuring acute and chronic monophasic action potentials in vivo have been disclosed. Reference is made, for example, to U.S. Pat. No. 4,955,382 issued to Franz et al, U.S. Pat. No. 4,690,155 issued to Hess, U.S. Pat. No. 5,425,363 issued to Wang, and U.S. Pat. No. 6,152,882 issued to Prutchi. These disclosed methods generally include the use of a wire conductor for conducting an electrical signal sensed by a sensing electrode. As indicated above, an electrical signal sensed by a sensing lead or catheter utilizing an electrical wire conductor will generally be subject to electrical noise and artifacts.
Another limitation of metallic wire based leads is that they are generally incompatible with magnetic resonance imaging because the magnetic field can induce unwanted current in metallic wire conductors. In addition, an implanted metallic wire based lead can become dislodged by the strong magnetic field. Because MRI examinations are prescribed for a variety of diagnostic purposes, it is desirable to provide implantable medical sensing leads that are MRI compatible.